That means if he lifts up a tank, he might show effort in doing so, or if he is going to push a small planet out of its orbit, he will show the same level of effort.

The mysterious power level is a direct result of the cosimic rays from Directors and writers who don't pay attention to, or even give much thought to Superman's power level, so they keep it vague as it needs to be for any given scene to work.

This is similar to scale in Transformers cartoons, this is size on convenience.

Same concept.

This.

Good example from comics would be All-Star Superman:

#1- After supercharge can lift 5 quintillion tons at the lower levels of his power.

#9- Beaten up by standard power Kryptonians.

#11- Struggles to defeat Solaris, with help from S-Robot Army.

#12- Cannot defeat Lex Luthor with standard Kryptonian power level, yet by the end of the issue demonstrates a feat of power greater than any other.

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*\S/T*"But that's the thing about Batman. Batman thinks of everything.""There's always a way. When the odds are impossible- do the impossible."